Site Designer could not find the file in the current Web application. Usually this is the result of entering an incorrect file extension for a site diagram page, such as .gif. Site Designer can only create new .htm or .asp files. If a file extension other than .htm or .asp is entered, Site Designer attempts to match the file name to an existing file in the project. If Site Designer cannot match the file name, Site Designer assumes the file does not exist.
To correct this error, use an .htm or .asp extension for the file.
Specify a file that exists in the Web application.
For more information, see https://msdn.microsoft.com/en-us/library/aa232966(v=vs.60).aspx .